1. Field of the Invention
The present invention relates to a dry process for forming metal patterns on a surface where a depolymerizable polymer is used as a lift-off or release layer for a blanket metallization layer.
In a preferred embodiment, the depolymerizable polymer is blanket coated onto the surface, a reactive ion etching (RIE) etch stop barrier is formed on the depolymerizable polymer and patterns created through the polymer and etch stop barrier using a combination of lithographic and RIE techniques. Blanket metal is vapor deposited onto the surface thereby filling in the etched pattern. After metal deposition, the sample is heated to depolymerize the polymeric release layer, thereby destroying the adhesion of the blanket metal-etch stop barrier-polymer laminate to the surface. Upon cooling, the metal-RIE etch barrier-polymer laminate is readily removed leaving behind the well-adhered metal pattern deposited through the pattern.
2. Description of the Prior Art
IBM Technical Disclosure Bulletins Vol. 22, No. 2, July 1979, pages 541 and 542 disclose, respectively, the use of gamma radiation treatment to enhance the unzipping of poly-.alpha.-methylstyrene and the use of poly-.alpha.-methylstyrene and polymethylmethacrylate as unzippable polymers for green sheets.
IBM Technical Disclosure Vol. 15, No. 1, June 1972, page 174, discloses inter alia, polymethylmethacrylate electron beam resists where heating is used to bring out the image rather than developing in a solvent.
U.S. Pat. No. 3,985,597 Zielinski discloses a wet lift-off system based upon the use of a solvent where polysulfone is essentially dissolved out in N-methyl pyrrolidone.
U.S. Pat. No. 4,004,044 Franco et al discloses a wet method for forming patterned films utilizing a transparent lift-off mask where a photoresist layer is removed in a solvent such as N-methyl pyrrolidone.
U.S. Pat. No. 4,181,755 Liu et al discloses a wet method for generating thin film patterns by an inverse lift-off technique which involves soaking in an appropriate photoresist solvent.
U.S. Pat. No. 4,224,361 Romankiw discloses a high temperature lift-off technique.
U.S. Pat. No. 4,272,561 Rothman et al discloses a solvent-based wet lift-off technique used in the fabrication of integrated circuits.
U.S. Pat. No. 4,328,263 Kurahasi et al discloses a process for manufacturing semiconductor devices where a solvent-based wet lift-off technique is used.
The process of the present invention finds application in any of the above processes where a metallization layer is removed except, of course, no solvent lift-off technique is involved in the present invention.
The present invention avoids the disadvantages of currently used wet procedures which involve solvent exposure to remove organic masking layers which can lead to device solvent contamination and adhesion loss with polymer containing devices and structures.
For example, per the prior art wet procedures if a lift-off layer comprises a polysulfone, the same is typically removed by soaking in hot N-methyl pyrrolidone (NMP) for 8-18 hours at elevated temperatures, e.g., 85.degree.-130.degree. C. The solvent dissolves the polymeric polysulfone layer and removes the overlying metal film, leaving behind the desired metallization pattern, e.g., metal vias. Certain devices employ organic insulators in the structure such as a polyimide in place of the inorganic insulators. Problems encountered with this particular system using polysulfone as a lift-off layer include: (1) long soak times in hot NMP; (2) moderately polar, strong solvents such as NMP readily swell organic polymers such as polyimides, the resulting volume increase stressing and cracking barrier layers such as Si.sub.3 N.sub.4 ; (3) polymers such as polyimides readily absorb NMP even at room temperature, which are then extremely difficult to remove from the device; and (4) polysulfone is sensitive to electron beam radiation which apparently leads to cross-linking, resulting in reduced solubility and requiring longer NMP soak times to effect lift-off.
Polysulfones are also moisture sensitive, and their characteristics change with humidity, a problem not encountered with the depolymerizable polymers of the present invention.